High speed lapping machine and method

ABSTRACT

To maintain high quality production while lapping at machine speeds up to twice conventional operational rates, a lapping type of machine is improved to reduce or eliminate unwanted periodic changes in driving moments between driving and driven gears being lapped by inserting dynamic system modifiers, in the form of energy storing or isolating devices, at critical points between the driven gear and the remaining driven elements of the machine. Examples of this improvement include insertion of an elastomeric coupling in a portion of the drive train between a driven gear and a braking means of a lapping machine; provision for an arbor assembly for the driven gear which contains spring elements to permit limited rotational movements between the gear mounting elements of the arbor and the rest of the arbor assembly; and utilization of a pulley assembly which includes one or more spring devices between two major housing elements of the pulley assembly.

United States Patent [191 Ellwanger et al.

[ Apr. 30, 1974 Primary ExaminerDonald G. Kelly Attorney, Agent, or Firm-Ralph E. Harper ABSTRACT To maintain high quality production while lapping at machine speeds up to twice conventional operational rates, a lapping type of machine is improved to reduce or eliminate unwanted periodic changes in driving moments between driving and driven gears being lapped by inserting dynamic system modifiers, in the form of energy storing or isolating devices, at critical points between the driven gear and the remaining driven elements of the machine. Examples of this improvement include insertion of an elastomeric coupling in a portion of the drive train between a driven gear and a braking means of a lapping machine; provision for an arbor assembly for the driven gear which contains spring elements to permit limited rotational movements between the gear mounting elements of the arbor and the rest of the arbor assembly; and utilization of a pulley assembly which includes one or more spring devices between two major housing elements of the pulley assembly.

7 Claims, 8 Drawing Figures 1 HIGH SPEED LAPPING MACHINE AND METHOD [75] Inventors: Charles G. Ellwanger; Gary J.

Kimmet; Robert F. Pigage, all of Rochester, NY.

[73] Assignee: The Gleason Works, Rochester,

[22] Filed: June 26, 1972 [21] Appl. No.: 266,081

[52] US. Cl 51/26, 51/287, 51/317 [51] Int. Cl B24b 21/16 [58] Field of Search 51/26, 287, 317

[56] References Cited UNITED STATES PATENTS 3,293,805 12/1966 Davis 51/26 X 3,176,512 4/1965 Hediger 51/26 X 3,099,901 8/1963 Hunkeler.... 51/26 2,919,518 1/1960 Bauer 51/26 2,947,120 8/1960 Bauer 51/26 3,069,813 12/1962 Bauer 51/26 2,691,250 10/1954 McMullen 51/26 I2 I 1 l4? PATENTED 30 IBM SHEEI 3 BF '4 FATENTEDAPRBO 197': I 3807.094

SHEET u UF 4 1 HIGH SPEED LAPPING MACHINE AND METHOD BACKGROUND AND BRIEF DESCRIPTION OF INVENTION This invention relates to improvements in machines which are capable of running a pair of gears in meshing engagement for the purpose of carrying out an automatic finishing treatment of the pair of gears. More specifically, the invention is concerned with an improvement in a gear lapping machine which permits such machines to be adapted to relatively high speed lapping operations, in the range of 2,400 to 2,800 rpm (as compared to prior art lapping operations at about 1,200 rpm) without costly modification or redesign of the machines.

It is known in this art to provide machines which will run a pair of gears together in meshing relationship for the purpose of carrying out a treatment of the pair of gears to produce a matched gear set as a finished product. Typically, such machines provide for a mounting of a first gear on a first spindle means connected to a driving motor which functions to transmit a constant driving moment to the first spindle means during a lapping operation, while a second spindle means carries a second gear for meshing engagement with the first gear, the second spindle means being operatively connected to a braking means which functions to apply a constant braking moment to the second spindle means during the lapping operation. With this arrangement, the two gears can be run together with a lapping compound applied to their surfaces to produce a matched gear set in which the two gears are highly compatible for applications requiring uniformity of motion transmission between tooth bearing surfaces when one gear is used to drive the other gear. Examples of known lapping machines in this art are described in U. S. Pat. Nos. 2,691,250; 2,919,518; 2,947,120; 3,069,813; and 3,099,901.

Known machines of the type just described have been operated at spindle speeds of about 1,200 rpm to carry out lapping operations. Certain improvements in design have been made in such machines to permit a decrease in lapping time by increasing spindle speed to about 2,400 rpm and as high as 2,800 rpm. However, such relatively high speeds of operation have produced, at times, unexpected variations in tooth bearing patterns for the gears being treated by the machines. The type of variation in tooth bearing patterns which is created is considered to be undesirable, and machine designs and operations have been investigated to attempt to determine methods and means by which the lapping operation can be carried out at such relatively high speeds with greater consistency of bearing patterns for the tooth surfaces of the gears which are being treated.

1t is-known that dynamic machine forces do not increase linearly but rather in proportion to the square of the exciting displacement frequency. Therefore, investigation of this subject suggests that very high speed lapping operations can be expected to rapidly magnify extremely slight imperfections in certain running components of a lapping machine and it is believed that imperfections in the anti-friction bearings which support the driving spindle (usually referred to as the pinion spindle) can account for a type of exciting displace:

ment frequency which can be magnified by very high speed operations to a point of producing unsatisfactory lapping of a pair of gears. These imperfections in the spindle are extremely small and appear in spindle constructions which are as nearly perfect as can be produced by present day manufacturing techniques. Thus, it is not feasible, in a practical sense, to attempt to improve upon tolerances of spindle assemblies available for such machines, although it would be possible to use relatively costly hydrostatic bearings for supporting such spindles for a more perfect rotation by a driving motor.

In accordance with the present invention, it is not necessary to utilize costly hydrostatic bearings or to otherwise modify existing lapping machines in a way that would be economically prohibitive or impossible. Instead, the machine is improved upon by interposing a relatively simple and inexpensive storing or isolating means in its drive train at a selected critical point between the driven gear of a gear pair and the braking means which is operatively connected to the spindle carrying the driven gear. The energy storing or isolating means acts to modify the dynamic system of the ma chine and may comprise an elastomeric coupling inserted in the aforesaid drive train or it may comprise improved arbor or pulley assemblies which include one or more spring devices for resisting relatively limited rotational movements between major components of such assemblies.

It should be noted that the discovery of the present invention is quite surprising in that it is the exact opposite of solutions traditionally utilized in precision machinery of this type in which stiffness is considered essential to accurate operation. That is, the use of a spring or elastomeric coupling would be expected to lead to a new problem of storing and releasing energy into a driven train in such a way as to create a new vibration or unwanted resonance. However, it has been discovered that the dynamic system modifiers of the present invention are completely workable because they reduce or eliminate unwanted periodic changes in driving moments, as initiated by imperfections in the driving spindle or other portions of the drive train of the machine, from developing and building up to such amplitude levels that tooth bearing patterns become non-uniform on the finished product. Thus, the devices of the present invention function, in a very real sense, to prevent a magnification to significant values of periodic changes in driving moments between a pair of meshed gears running together, even though there may exist rotational deviations of a much lower, and insignificant, amplitude as a result of periodic energy storage and release into the system from the elastomeric or spring devices inserted therein.

Dynamic machine forces are known to increase in proportion to inertia as well as in proportion to the square of the rotational speed, and so research leading to the present invention considered the effect of inertia in a drive train system experiencing unwanted changes in the forces acting between the driving and driven gears of a pair being lapped. It was discovered that reduction of inertia in the driven portion of the drive train also functions to help dampen unwanted changes in driving moments which are created in a system and, thereby, also helps prevent a magnification or reinforcement of forces that would result in undesirable tooth contact pattern distortions in the gears being treated by the system. Therefore, the invention disclosed herein contemplate that certain components of the driven portion of the machine can be reduced in mass or weight to maintain good quality products at very high operational speeds.

One embodiment of the present invention provides for insertion of a dynamic system modifier in the form of spring devices in a special arbor assembly which carries the second or driven gear of a pair of gears, in a lapping machine. This location for an energy storing or isolating means is theoretically desirable because it separates the gears being treated from the inertia of the substantial machine mass which comprises the driven portion of the drive train (i.e., the arbor itself, its spindle, and associated braking means for imparting a braking moment to the driven spindle.)

A second embodiment of the invention provides for an insertion of an elastomeric coupling in the second spindle of the machine which is operatively associated with a braking means and which carries the aforesaid arbor and second gear. This embodiment offers certain practical advantages over the previously mentioned arbor modification because of certain factors relating to cost of manufacture and corrosiveness of working environment associated with the arbor assembly itself.

A third embodiment provides for insertion of an isolating or dynamic system modifying device in the pulley means which connects the driven spindle to the broke system thru a belt.

These and other featuresand advantages of the present invention will become more apparent in the detailed discussion which follows. In that discussion reference will be made to the accompanying drawings as briefly described below.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagrammatic illustration of a drive train system for a typical lapping machine of the type contemplated by the present invention;

FIG. 2 is a side elevational view, in section, of an arbor assembly of a lapping machine which has been modified with the improvement of the present invention;

FIG. 3 is a front elevational view of the arbor assembly shown in FIG. 2;

FIG. 4 is a back elevational view, in greatly enlarged scale, of a portion of the arbor assembly of FIG. 2 as seen generally on lines 44 of FIG. 2;

FIG. 5 is a side elevational view in cross section, and in greatly enlarged scale, of a detail of the arbor assembly of FIGS. 2 4, as seen on line 5-5 of FIG. 3;

FIG. 6 is a side elevational view, partly in cross section, of a spindle assembly associated with the arbor for the driven gears showing an installation of an elastomeric coupling therein in accordance with another embodiment of the present invention;

FIG. 7 is an isometric view of a pair of mounting blocks used for securing the elastomeric coupling shown in FIG. 6; and

FIG. 8 is an exploded view of a braking pulley assembly of the gear spindle provided with energy storing or isolating means in accordance with a further embodiment of the present invention, with one part of the assembly shown in enlarged scale for clarity of disclosure.

DETAILED DESCRIPTION OF INVENTION FIG. 1 illustrates a typical drive train system for a lapping machine of the type which can be improved upon by the discoveries of the present invention. The basic arrangement of the illustrated drive train system is known in this art and does not constitute a separate invention.

The drive train system of FIG. 1 includes a first spindle means 10 for mounting a first gear 12 for rotation about a vertical axis. The gear 12 is typically referred to as a pinion gear in the type of gear relationship illustrated in FIG. 1 and is releasably secured to the spindle 10 in any known manner. The first spindle means 10 is operatively connected to a driving motor 14 through an output shaft 16 of the driving motor, a pair of pulleys and a drive belt 18, and an input shaft 20 of the first spindle means 10. In this way, the driving motor 14 functions to transmit a continuous driving moment to the first spindle means and the first gear 12 during a lapping operation.

A second spindle means is contained within a housing for rotation about a horizontal axis 22 and for mounting a second gear 24 in meshing engagement with the first gear 12. An arbor assembly 26 functions to carry the second gear 24 on the second spindle means, and known chucking and dechucking devices are provided for securing and releasing the second gear 24 relative the second gear carried thereby during a gear lapping operation. Braking moments are applied to the second spindle means through a drive belt 30 and a pair of pulleys which includes a braking pulley assembly 32.

As discussed above, it is believed that imperfections in the first spindle means 10account for a generation of unwanted changes in driving moments between the gear 12 and the gear 24, resulting in undesirable changes in tooth bearing patterns formed on the two gears during a high-speed operation. Instead of attempting to further perfect or modify the first spindle means 10 to eliminate even extremely minute imperfections which contribute to an unsatisfactory lapping operation, the present invention provides for a reduction or elimination of unwanted periodic changes in driving moments between the first and second gears by inserting energy storing or isolating means in the drive train at one or more selected and preferred points in locations between the driven second gear 24 and its associated braking means 28. Separate embodiments of the invention can be utilized at one of three preferred locations in the driven portion of the drive train which includes the relatively large mass of the second spindle means 23 and its associated structures. Thediscussions which follow will describe the various embodiments of this invention with reference to separate views of the drawings which can be related to the overall drive train system illustrated in FIG. 1.

FIGS. 2 5 relate to an embodiment of the present invention which provides for a modification of the arbor assembly 26 of FIG. 1 so as to isolate the substantial masses of the second spindle means and its associated structures from the second gear 24 as it is driven by the first gear 12.

FIG. 2 illustrates basic components of the improved arbor assembly 26 as including a first section 40 and a second section 42. These two basic sections of the arbor assembly are related to one another so that driving moments are imparted by the section 40 to the section 42 and braking moments are transmitted back from the section 42 to the section 40. One or more spring means 44, which will be discussed in greater detail with reference to FIGS. 3 5, are interposed between the sections 40 and 42 so that driving and braking moments are actually transmitted through the spring means. Thus, the spring means can function to minimize forces acting between the meshing teeth of the gears being lapped, since small periodic changes in driving moments received by second gear 24 are stored temporarily in the spring means 44 and are not transmitted to the remainder of the drive train, thereby apparently preventing the build up of any undesirable large changes in the driving moments as referred to above.

Considering the structures of FIG. 2 in greater detail, it can be seen that the first section 40 is formed from an inner backing ring 46 and an outer slinger ring 48 (also, see FIG. 5) secured together to define a component which can be mounted on an arbor sleeve 50. The arbor sleeve 50 is carried on a centering unit 52. The centering unit 52 is mounted on an actuating rod 53 of an expander unit 54 for chucking and duchucking the gear 24 relative to the arbor assembly.

The second section 42 of the arbor assembly is fastened to an end of a spindle section 55 mounted within a housing. The spindle 55 is mounted for rotation relative to the housing, and braking moments are applied to an opposite end section of the spindle.

FIGS. 3 5 illustrate details of the spring means 44 and the manner in which they are mounted between the first section 40 and second section 42 of the arbor as sembly shown in FIG. 2. FIG. 3 shows that the spring means 44 are carried in positions 180 apart to provide for an energy absorbing or isolating means which is effective for either direction of relative rotation between the two sections 40 and 42 of the arbor assembly. Thus, if during a lapping operation there is a small increase or decrease in driving moments between the two gears 12 and 24, such changes will not be imparted to the substantial mass of the second spindle means and its associated structures. As shown in FIGS. 4 and 5, the first section 40 of the arbor assembly carries a driving pin means 60 which is secured at a fixed point to the driving section 40 so as to intercept the central axis of the spring means 44. The spring means 44 is carried within a bore 62 formed in the second section 42, and when compressed, the spring means 44 bears against a plunger and rod assembly 64 which makes actual contact with the drive pin means 60 of the driving section 40. A spring retainer bolt 66 is threaded into a threaded end of the bore 62 to hold the spring 44 in place and to guide movement of the rod portion of the plunger and rod assembly 64. With this arrangement, it can be seen that limited rotational movement is available between the two arbor assembly sections 40 and 42. Sufficient rotational differential between the two sections will result in one of the drive pin means 60 contacting its associated plunger 64 to compress a spring means 44, thereby storing energy from any unwanted changes in driving moments between the two arbor sections. This energy is released at a later time with no significant impact upon a satisfactory running relationship between the gears 12 and 24.

Another feature of the arbor assembly illustrated in FIGS. 2 5 is to provide for a stop pin means 68 carried in a fixed location in the second section 42 of the arbor assembly. The stop pin 68 (see phantom line illustration in FIG. 4) is positioned to normally maintain the plunger 64 out of contact with the drive pin means 60 when the arbor is at rest or when it is running in a neutral condition without any aberrations in driving moments between the two sections. This prevents a binding of the two sections 40 and 42, which might otherwise result from a continuous contact between the drive pin means 60 and the plunger head 64, and this improves speed and efficiency of operation of the energy storing or isolating devices of the arbor assembly.

A still further feature of the arbor assembly of FIGS. 2 5 is that the degree of compression of the springs 44 can be adjusted by adjusting the axial position of the retainer bolt 66. This is accomplished by threading the retainer bolt 66 inwardly or outwardly relative to the threaded bore into which it is received.

FIGS. 6 and 7 illustrate another embodiment of the present invention. This embodiment provides for insertion of an energy absorbing or isolating means in the form of an elastomeric coupling 70 at some point along the length of the second spindle means of the machine illustrated in FIG. 1. This embodiment provides for a reduction or elimination of unwanted periodic changes in driving moments between the gears 12 and 24 without a redesign and rebuilding of the arbor assembly 26.

The elastomeric coupling 70 comprises a known type of elastomeric element which is typically used in flexible coupling constructions for providing a range of torsional and axial deflection in a drive train. Such elements can be manufactured from rubber or synthetic elastomeric materials which offer resistance to lubricants and other corrosive environments in which they may be utilized. As shown in FIG. 7, the elastomeric coupling 70 is mounted between a pair of mounting blocks 72 and 74. Opposite end faces of the elastomeric coupling 70 are bolted and secured to respective end walls of the mounting blocks with the two mounting blocks being spaced apart to permit limited rotational deflection therebetween. In a preferred arrangement, each mounting block 72 and 74 comprises a cupshaped element carrying axially extending finger portion 76 which is received within slot 78 formed between corresponding finger portion of an opposed mounting block. The relationship between the finger portion 76 and slot 78 is such. that limited rotational movement can take place between the two mounting blocks 72 and 74, but extreme deflections will be stopped by a contact between edges of the extending portion 76 and slot 78. Of course, a single stop limit means may be provided between the two mounting blocks if desired.

The mounting block 72 is secured to a cap element 80, which in turn is secured to a housing member 82. The housing member 82 is keyed to a front spindle element 84 so that braking moments are transmitted through the flexible coupling 70 and to the arbor assembly 26. The other mounting block 74 is keyed to a back spindle element 86, and the back spindle element carries a braking pulley 32 for receiving braking moments from the braking means 28. Any aberrations in rotational moments between the mounting blocks 72 and 74 of the elastomeric coupling 70 are absorbed by a twisting of the coupling element between the two mounting blocks.

FIG. 6 also illustrates a chucking and dechucking means for operating an expander element carried by the arbor assembly 26. As with previous chucking and dechucking arrangements, a control rod member 88 is spring urged to maintain the gear 24 in a secure mounted position on the arbor assembly 26. However, since it is preferred to mount the flexible coupling 70 in the drive train without applying any tension or compression to the elastomeric material of the coupling, it is necessary to depart from known chucking and dechucking arrangements to provide for actuation of the control rod 88. This is accomplished by providing a piston element 90 in a chamber portion 92 within the housing 82 so that hydraulic fluid can be introduced behind the piston element 90 in the chamber portion to cause the piston element to advance (toward the left in FIG. 6) and move the control rod 88 to a dechucking position. A release of hydraulic fluid from the chamber 92 allows the control rod 88 to be urged back to a chucking position by the action of spring elements 94 carried between an end wall of the housing 82 and the piston member 90. Chucking and dechucking movement of the control rod 88 are carried out without application of any compression or tension forces to the elastomeric coupling 70.

FIG. 8 illustrates a third embodiment of the invention as applied to a modified pulley construction which can be substituted for the braking pulley 32 of the drive trainsystem illustrated in FIG. 1. The modified pulley assembly includes two major housing elements 100 and 102 which are assembled so as to provide for a split pulley assembly offering limited rotation between its two housing elements. In the illustrated embodiment, element 100 is placed over the housing element 102 by insertion of a sleeve portion 104 of the housing element 102 into a cylindrical bearing ring 106 carried by the housing element 100. A thrust bearing 108 is inserted over the sleeve portion 104 prior to assembly of the two major housing elements so that opposite faces of the thrust bearing 108 contact a face of the housing element 1-02 and a face of the cylindrical bearing ring 106 of the housing 100. After this, a pulley sleeve 110 is press fitted over the outside surface (not shown) of the cylindrical bearing carried by the cylindrical bearing ring 106. Assembly of the elements shown in FIG. 8 places two energy absorbing devices on opposite sides of an axis of rotation 112 of the assembly. One of the energy absorbing devices is shown removed from a bore into which it is secured in the assembly and is illustrated in enlarged scale for clarity of understanding. This device includes a housing cylinder 114 having a bore therein for holding a spring 116. A plunger and rod assembly 118 are inserted within an open end of the bore to contact the spring 116, and the housing cylinder 114 is screw threaded into a threaded bore 120 of the housing element 100. A similar arrangement is provided on an opposite side of the housing element 100. When assembled, each of the plunger and rod assemblies contact fixed dowel elements 122 secured to the housing element 102. The purpose in providing a curved surface of contact between the energy absorbing devices of the housing element 100 and fixed surfaces of the housing 102 is one of preventing a binding of the two separate housing elements as might be the case if flat surfaces were repeatedly impacted with one another.

The split pulley assembly of FIG. 8 offers a feature of adjustment of the degree of compression applied to the spring elements 116. This is accomplished by rotating each housing cylinder 1 14 inwardly or outwardly of its bore 120.

Any of the above described embodiments can be further modified to reduce mass in the drive train system so as to reduce inertia magnifications in the system. For example, components of the improved arbor assembly of FIG. 2 can be formed from aluminum or other light weight materials to reduce inertia in the overall drive train system. Similar changes can be made with the arbor assembly used with the FIG. 6 embodiment of the invention. In fact a preferred utilization of the FIG. 6 concepts provides for a low inertia arbor assembly and a'reduced diameter (low inertia) spindle assembly for use with the flexible coupling 70.

Although this invention has been described with specific reference to improvements in high speed lapping machines, it should be apparent that its principles can be applied to other drive train systems which require a control of turning moments between various portions of the systems.

What is claimed is: I

1. An improved machine capable of running a pair of gears in meshing engagement at relatively high speeds for carrying out a finishing treatment, such as lapping, of the pair of gears, comprising a drive train having:

a first spindle means for mounting a first gear for rotation, said first spindle means being operatively connected to a-driving motor which functions to transmit a driving moment to said first spindle means,

a second spindle means for mounting a second gear in meshing engagement with said first gear, said second spindle means being operatively connected to a braking means which functions to apply a braking moment to said second spindle means during a gear treating operation, and

means including at least one dynamic system modifier for reducing or eliminating unwanted periodic changes in driving moments between said first and second gears during a running engagement of the two gears in a treating operation, said dynamic system modifier being interposed, in said drive train, between a driven gear of said two gears and its associated braking means.

2. The improved machine of claim 1 wherein said dynamic system modifier comprises an energy storing or isolating means.

3. The improved machine of claim 2 wherein said energy storing or isolating means comprises an elastomeric coupling.

4. The improved machine of claim 3 wherein said elastomeric coupling is provided with stop limit means to limit its torsional deflections.

5. The improved machine of claim 4 wherein said elastomeric coupling is mounted between a pair of mounting blocks which carry means for limiting torsional deflections of the elastomeric coupling, said elastomeric coupling being interposed in said drive train without being axially compressed or tensioned.

6. The improved machine of claim 5 and including a chucking and dechucking means in said second spindle means for chucking and dechucking said second gear chucking and dechucking functions without compressing or tensioning said elastomeric coupling.

7. The improved machine of claim 1 wherein said means for reducing or eliminating said unwanted changes in driving moments includes low inertia assemblies interposed in said drive train between said second gear and its associated braking means. 

1. An improved machine capable of running a pair of gears in meshing engagement at relatively high speeds for carrying out a finishing treatment, such as lapping, of the pair of gears, comprising a drive train having: a first spindle means for mounting a first gear for rotation, said first spindle means being operatively connected to a driving motor which functions to transmit a driving moment to said first spindle means, a second spindle means for mounting a second gear in meshing engagement with said first gear, said second spindle means being operatively connected to a braking means which functions to apply a braking moment to said second spindle means during a gear treating operation, and means including at least one dynamic system modifier for reducing or eliminating unwanted periodic changes in driving moments between said first and second gears during a running engagement of the two gears in a treating operation, said dynamic system modifier being interposed, in said drive train, between a driven gear of said two gears and its associated braking means.
 2. The improved machine of claim 1 wherein said dynamic system modifier comprises an energy storing or isolating means.
 3. The improved machine of claim 2 wherein said Energy storing or isolating means comprises an elastomeric coupling.
 4. The improved machine of claim 3 wherein said elastomeric coupling is provided with stop limit means to limit its torsional deflections.
 5. The improved machine of claim 4 wherein said elastomeric coupling is mounted between a pair of mounting blocks which carry means for limiting torsional deflections of the elastomeric coupling, said elastomeric coupling being interposed in said drive train without being axially compressed or tensioned.
 6. The improved machine of claim 5 and including a chucking and dechucking means in said second spindle means for chucking and dechucking said second gear against an arbor carried by said second spindle, said chucking and dechucking means having spring means for normally urging the chucking and dechucking means into chucking engagement with a gear, and including hydraulic means for moving said chucking and dechucking means in a dechucking direction to disengage its contact with a gear, said spring means and said hydraulic means both being arranged to carry out chucking and dechucking functions without compressing or tensioning said elastomeric coupling.
 7. The improved machine of claim 1 wherein said means for reducing or eliminating said unwanted changes in driving moments includes low inertia assemblies interposed in said drive train between said second gear and its associated braking means. 